The present invention relates to a dry roughing vacuum pump that allows electrical power consumption to be reduced. It relates in particular to roughing vacuum pumps of the “dry rotary lobe pump” type, such as lobe pumps of the “roots” type, claw pumps, scroll pumps, screw pumps, piston pumps, etc., in a single-stage or multi-stage version.
The electrical power needed for compressing gases is one of the significant parameters involved in the power consumption of dry roughing vacuum pumps. This compression power is used mainly in the last two compression stages in the case of a multi-stage pump of the roots or claw type, and in the last flights in the case of a screw pump.
In order to reduce the electrical power consumption of the roughing vacuum pump, one known solution is to lower the pressure in the final compression stage using an ejector. The ejector operates on the principle of the Venturi effect. It means that a drop in pressure can be obtained from the injection of a compressed fluid, such as a gas like nitrogen or compressed air for example, at a narrowing of the passage for the gases. A depression is thus created without any direct consumption of electrical power.
However, positioning the ejector in the discharge line reduces the conductance for the passage of the pumped gases so that significant flows of gas that occur, for example, when roughing a vacuum in the chamber, can no longer be absorbed.
A solution known from document FR 2952683, is to mount the ejector in a parallel circuit, arranged to bypass the nonreturn valve. Thus, when the nonreturn valve is closed, the gas follows the bypass circuit in which the ejector is mounted. The injection of motor gas into the narrowing of the bypass circuit produces a lowering of the pressure on the discharge side and therefore a drop in the consumed power. Further, in the event of a surplus of gas, the nonreturn valve opens, short-circuiting the parallel circuit.